The centroid or the centroid axis of a washing machine drum is displaced from the rotational axis (out of the rotational center in an ideal initial position) by nonuniform distribution of the laundry and is therefore spaced apart from the rotational axis. The rotation of the centroid with the spacing from the rotational center causes centrifugal forces on the drum which have to be supported by the bearing arrangement on the washing machine. The greater the centroid axis deviates from the initial position, the greater what is known as the unbalance. This unbalance leads to undesired movements of the washing drum which is usually suspended elastically. The drum can swing out so far from the ideal rotational axis that contact occurs between the washing machine and the drum. Moreover, the rotational axis of the drum cannot coincide with the main axis of inertia of the drum as a result of the nonuniform distribution of the laundry in the drum, that is to say the rotational axis is tilted at the centroid. Dynamic unbalances of this type cause bending moments on the bearing journals during operation of the washing machine, by way of which bearing journals the drum is mounted rotatably in the washing machine, and result in undesired oscillations or in unnecessary bearing stresses.
The high bearing forces and oscillations which result from the unbalances can lead to premature wear of the bearing arrangements.
The movement and flooding of the is not sufficient in an overfilled washing drum. The cleaning action is insufficient. On the other hand, a washing drum which is not filled sufficiently leads to unnecessary energy and water consumption. In modern washing machines, the filling ratio, that is to say the amount of dry laundry in relation to the drum volume, is a characteristic variable for calculating washing processes which are optimized in terms of consumption and results. The amount is determined in the washing machine by means of a device using the dry weight. Subsequently, the operating criteria are fixed automatically as a function of the filling ratio which is calculated therefrom.
Positional changes are therefore to be understood as all changes of the position of the centroid or the centroid axis of the drum, and therefore also at least of one bearing for the drum, with respect to the initial position of the rotational axis in the case of nonstressed and nonrotating drums. Unbalances as a result of nonuniform material distribution on the drum itself are therefore also considered.
DE 31 17 106 A1 describes a measuring device of the generic type, by way of which the oscillation amplitudes which result from harmful oscillations and therefore the positional change of the drum with respect to the rotational axis are measured. The optical sensory array of the measuring device is arranged between the drum and one tub of the washing machine and outputs a signal after a limiting value is exceeded of the closeness of the drum and the tub. The signal causes a change and regulation of the rotational speed of the drum, with the target of reducing the unbalances. The measuring device is not suitable for detecting positional changes of the drum by means of the weight of the drum or for continuous measuring operations.